Variable displacement internal combustion engines are conventionally referred to as split engines and have heretofore incorporated valve deactuators for selectively deactuating selected cylinders during periods of low power demand. All of the cylinders are allowed to operate by the valve deactuators during periods of high power demand so as to provide the engine power output required. Operation of the valve deactuators in such a manner thus provides fuel efficiency without sacrificing performance.
Automatic operation of a variable displacement internal combustion engine requires a control system that is capable of sensing the engine power demand and properly operating the valve deactuators in response thereto so as to result in fuel economy without sacrificing performance. One such prior control system is disclosed by U.S. Pat. No. 4,173,209, entitled "ENGINE CONTROL SYSTEM AND VALVE DEACTUATOR THEREFOR," filed Dec. 16, 1977 by Edgar R. Jordan as a divisional of U.S. patent application Ser. No. 815,743, now U.S. Pat. No. 4,175,534, also entitled "ENGINE CONTROL SYSTEM AND VALVE DEACTUATOR THEREFOR" and filed by Edgar R. Jordan on July 14, 1977, both of which prior patents are hereby incorporated by reference. The control system disclosed by the aforementioned patents includes a first sensor for generating a variable signal responsive to engine throttle position and a second sensor for generating a variable signal responsive to engine output speed. A comparator of the system compares the two signals and generates a variable output signal for controlling the valve deactuators.
Heretofore it has not been possible to sense intake manifold vacuum to operate valve deactuators of a variable displacement internal combustion engine due to the effect that deactuation of each cylinder has on the manifold vacuum. Dropping out one cylinder as the manifold vacuum drawn exceeds a predetermined extent then decreases the vacuum below the predetermined extent so that the cylinder again operates and again increases the vacuum and so on such as to result in an unstable on-off oscillation of the valve deactuator and the associated cylinder. Such an oscillatory operation does not promote engine efficiency or smooth engine operation.